The present invention relates to a method for increasing the photosensitivity of optical fibers at rates several times faster than those accomplished by prior methods. Specifically, the present invention comprises a method for rapidly diffusing hydrogen or deuterium into silica glasses to increase the photosensitivity of these glassy materials, and in particular of optical fibers.
Optical fiber-based devices are vital components in today's expanding high-volume optical communications infrastructure. Many of these devices rely on fiber Bragg gratings (FBGs) to perform light manipulation. An FBG is an optical fiber with periodic, aperiodic or pseudo-periodic variations of the refractive index along its length in the light guiding region of the waveguide. The ability to produce these refractive index perturbations in a fiber is necessary to manufacture FBGs and, hence, a number of optical components, such as optical sensors, wavelength-selective filters, and dispersion compensators.
Gratings are written in optical fiber usually via the phenomenon of photosensitivity. Photosensitivity is defined as the effect whereby the refractive index of the glass is changed by actinic radiation-induced alterations of the glass structure. The term "actinic radiation" includes visible light, UV, IR radiation and other forms of radiation that induce refractive index changes in the glass. A given glass is considered to be more photosensitive than another when a larger refractive index change is induced in it with the same delivered radiation dose.
The level of photosensitivity of a glass determines how large an index change can be induced in it and therefore places limits on grating devices that can be fabricated practically. Photosensitivity also affects the speed that a desired refractive index change can be induced in the glass with a given radiation intensity. By increasing the photosensitivity of a glass, one can induce larger index perturbations in it at a faster rate.
The intrinsic photosensitivity of silica-based glasses, the main component of high-quality optical fibers, is not very high. Typically index changes of only .about.10.sup.-5 are possible using standard germanium doped fiber. However, it has been observed that by loading the glass with molecular hydrogen before irradiating it with actinic radiation, one can increase tremendously the photosensitivity of the glass. Index changes as large as 10.sup.-2 have been demonstrated in hydrogenated silica optical fibers.
An early reference to an increase in photosensitivity due to exposure to hydrogen may be found in D. McStay, "Photosensitivity Changes in Ge-Doped Fibers Observed by Raman Spectroscopy", SPIE, Vol. 1314, Fibre Optics 1990. A peak was observed for samples of a Coming 1521 fiber reported to have been immersed in a hydrogen bath at varying pressure, times and temperatures up to 150.degree. C. An exemplary reported exposure consisted of a fiber treated at 1 atmosphere and 24.degree. C. for 3 days. The fibers exhibited a weak photosensitive reaction.
In F. Ouellette et al., "Permanent Photoinduced Birefringence in a Ge-doped Fiber", Applied Physics Letters, Vol. 58, p. 1813, Apr. 29, 1991, reports an attempt to increase photosensitivity by hydrogen exposure at relatively high temperatures. Fiber strands having a core doped with germanium were put in a pressure chamber with 12 atm H.sub.2 and heated at 400.degree. C. for 4 hours. The total index change for the hydrogen-treated fiber was estimated to be close to 10.sup.-5. G. Meltz et a., SPIE, Volume 1516, International Workshop on Photoinduced Self-Organization in Optical Fiber, May 10-11, 1991, paper 1516-18 reports treating a doped germanosilicate preform rod for 75 hours at 610.degree. C. in 1 atm H.sub.2. Such high-temperature exposure later was found to cause high optical loss in the fiber, usually rendering the fiber useless. U.S. Pat. Nos. 5,235,659 and 5,287,427 discuss a method for exposing at least a portion of a waveguide at a temperature of at most 250.degree. C. to H.sub.2 (partial pressure greater than 1 atmosphere (14.7 p.s.i.), such that irradiation can result in a normalized index change of at least 10.sup.-5 . U.S. Pat. No. 5,500,031, a continuation-in-part of the above-mentioned '659 patent, speaks of a method of exposing the glass to hydrogen or deuterium at a pressure in the range of 14-11,000 p.s.i. and at a temperature in the range 21-150.degree. C. The parameters described in these references are probably most typical for hydrogen-loading an optical fiber
The '031, '659 and '427 references point out problems with hydrogen loading methods in which temperatures exceed 250.degree. C., or even 150.degree. C. In teaching away from such references, the '659 Patent indicates that at high-temperatures "typical polymer fiber coatings would be destroyed or severely damaged" (column 1, lines 51-54). It further emphasizes the fact that "the prior art high temperature sensitization treatment frequently increases the optical loss in the fiber and/or may weaken the fiber" (column 1, lines 54-56). Finally, the '659 patent differentiates itself from the prior art by stating that a high temperature treatment involves "a different physical mechanism" than does a low-temperature treatment.
To achieve the desired level of hydrogen in fiber with conventional hydrogenating methods (.about.1 ppm), one will typically expose fiber to a hydrogen atmosphere for several days and, in some cases, for several weeks. Exemplary exposures such as 600 hours (25 days), 21.degree. C., at 738 atm or 13 days, 21.degree. C. at 208 atm are reported as typical. Obviously, such long exposures extend the time required to fabricate optical devices that rely on photosensitive glass. Because of the long duration needed for traditional fiber hydrogenation, several pressure vessels are needed in a high-volume production environment to increase throughput and avoid idle time. These vessels are costly to install safely and increase the potential for serious accidents, especially when multiple vessels with separate control valves and gas supply cylinders are involved. Although installing multiple vessels can increase production throughput, the hydrogenation process hampers grating fabrication cycle time, thus new product and specialty product development time can be compromised severely.
The need exists for a more time-effective method for increasing the photosensitivity of glassy materials.